It shouldn't matter, velvet's life cycle is 48 hours. So if it makes it through 14 days, it survived several life cycles, it seems reasonable that 30 days would not be an issue. The 14 days is only acceptable if you transfer to a completely sterile quarantine post-treatment. If you're keeping in a single QT for the duration, 30 days is required. This is more for ich than velvet, however.
Is it 48 hours at 78F? According to this paper, 14 days is the minimum to control an outbreak of the infection - but they claim its life cycle is 3-6 days @ 20c.
http://agrilife.org/fisheries/files...mportant-Parasite-of-Cultured-Marine-Fish.pdf